The panel board industry, includes, but is not limited to, plywood, OSB (Oriented Strand Board) (commonly referred to as flake or wafer board), medium density fiber board, particleboard, and other products, inclusively referred to herein as lignocellulosic composite products. In each of these composite products and in lumber (the wood of trees cut and prepared for use as building material) (collectively referred to herein as “lignocellulosic products”) it is desirable to control the water absorption or “uptake” and swelling, both of which have detrimental affect on the utility of the product. For example, in plywood used for floor underlay, swelling causes buckling or creep in the final wood or tile overlay. Similar problems occur with swelled OSB used as a roofing member applied to areas which will experience moisture. These composite board panels, like wood and other lignocellulosic products, are also known to deteriorate on the job site due to open storage, as a result of water uptake, which leads to biological degradation resulting from the growth of, and infestation by, bacteria, fungi, and insects.
Lignocellulosic composite products are conventionally manufactured by hot pressing lignocellulosic materials with wax and thermosetting resin. This is referred to as a conventional bonding process. The wax is a sizing agent to improve the water resistance of the composite. The resin is a bonding agent that holds the materials comprising the composite together, thus forming them into a unitary shape. Resoles are commonly used as the binding resin for lignocellulosic composite products.
In the conventional hot press method of manufacture of lignocellulosic composite products, a lignocellulosic material is combined with a phenolic resin and other components in a blender or mixer. The blend or mixture that results is pressed, typically under pressures above atmospheric and temperatures greater than room temperature, to produce the composite. Lignocellulosic materials used in the production of mats may be selected from the group consisting of wood fiber, wood flake, wood strands, wood chips and wood particles, and mixtures thereof. The lignocellulosic materials listed here are referred to in the art as wood furnish. However, it is well known that other wood furnish, such as straw, bagasse, wood bark, recycled wood fiber, recycled paper fiber, and mixtures thereof, may also be used. The wood furnish, once blended or mixed with the phenolic resin, is then formed onto a support material to make a pre-form in the approximate shape of the finished good. The pre-form is then placed on a caul plater in a hot press where the finished good is produced by applying pressures above atmospheric and temperatures greater than room temperature. The elevated temperatures and pressures cause the phenolic resin to polymerize, thus binding the pre-form into a unitary finished good. The hot press method is further described in U.S. Pat. No. 4,433,120 to Shui-Tung Chiu.
Lignocellulosic composite products primarily find use in construction or fabrication. These products may be used in building construction or any fabrication where wood is traditionally used. The poor dimensional stability of state-of-the-art lignocellulosic composite products affects their mechanical properties and reduces their load carrying ability. Another result of poor dimensional stability is unevenness of roof and floor underlayments, and of building siding. Two methods have been principally suggested as means to produce dimensionally stable lignocellulosic composite products. However, both of these methods have proven to be too costly to be used in practice. The first method is referred to as Bulking Treatment. In this method, lignocellulosic materials are impregnated with water-soluble polymers such as polyethylene glycol or impregnated with a low molecular weight resin such as phenol-formaldehyde, or with vinyl monomers and polymerized in situ. The second method is referred to as Chemical Modification. In this method, the lignocellulose may be esterified by, for example, acetylation, or it may be cross-linked using, for example, an aldehyde. An alternative method of Chemical Modification is to react hemicellulose with lignin under elevated temperatures, typically using steam treatment. These methods of chemical modification are costly and reduce the strength of the once-formed composite.
The phenol-formaldehyde resin used in the manufacture of lignocellulosic composite products may be in the form of a solid or a liquid. Powdered phenolic resins, such as novolac, resole, or combinations thereof, may generally be used. U.S. Pat. No. 4,098,770 to Berchem, et al., discloses a spray-dried phenol-formaldehyde resin modified with added non-phenolic polyhydroxy compounds, used in the manufacture of waferboard. Liquid phenol-formaldehyde resins, such as resole or resole and novolac combinations, may also be used in the manufacture of lignocellulosic composite products. Parameters for the manufacture of either liquid or solid phenol-formaldehyde resins are disclosed in Phenolic Resins, Chemistry, Applications and Performance, (A. Knop and I. A. Pilato, Springer-Verlag (1985)) and Advance Wood Adhesives Technology, (A Pizzi, Marcel Dekker (1994)).
Historically, molten hydrocarbon and simple emulsions utilizing stearic acid/triethanolamine (TEA) or diethanolamine (DEA) and/or lignosulfonate as surfactants have been used in the manufacture of board panels to impact water resistance with varying degrees of performance. However, they generate undesired emissions, inconsistent performance, handling and storage difficulties, foaming, and non-predictable application levels of the wax system to achieve the results.
A method widely used in the conventional bonding process to improve dimensional stability, as noted above, is the application of a wax sizing agent. The wax sizing imparts a certain degree of water repellency to the once-formed composite. Paraffin is a compound sizing agent. One method by which wax sizing imparts water repellency is by coating the surface of the lignocellulose, thus decreasing its surface tension. Another method by which wax sizing imparts water repellency is by partially filling the capillaries within the lignocellulose, thus providing a barrier to the capillary uptake of water.
Conventional preservatives for lignocellulosic products often contain heavy metals, for example, chromated copper arsenate (CCA). Pressure treatment of wood products, i.e., lumber, using CCA is referred to as wolmanizing. Other methods involve the use of creosote oil containing polycyclic aromatic hydrocarbons (PAHs), referred to as creosoting. In these conventional methods, the preservative will often penetrate the wood only around the edges. In addition, the use of a wolmanized and/or creosoted wood is coming under increasing pressure from environmental groups. Problems with CCA-treated wood include difficulty in meeting the Water Pollution Control Laws, the problem of waste wood treated with CCA or the like, at least in part due to the heavy metals present in this preservative. While creosote oil has good permeability, weather resistance and preservative property, it has problems such as odor, skin irritation, health damage, and a black color. Alternative preservative systems for lumber, with lower perceived risk, such as ammoniacal copper quat (ACQ), ammonial copper zinc arsenate (ACZA), copper bis(dimethyldithiocarbamate) (CDDC), ammoniacal copper citrate and copper azole, are also in limited commercial use.
Modern organic biocides are considered to be relatively environmentally benign and not expected to pose the problems associated with CCA-treated lumber, for example. Biocides such as tebuconazole are quite soluble in common organic solvents while others such as chlorothalonil possess only low solubility. The solubility of organic biocides affects the markets for which the biocide-treated wood products are appropriate. Biocides with good solubility can be dissolved at high concentrations in a small amount of organic solvents, and that solution can be dispersed in water with appropriate emulsifiers to produce an aqueous emulsion. The emulsion can be used in conventional pressure treatments for lumber and wood treated in such a manner can be used in products such as decking where the treated wood will come into contact with humans. Biocides, which possess low solubility, are often incorporated into wood in a solution of hydrocarbon oil such as AWPA P9 Type A, and the resulting organic solution is used to treat wood directly. Wood treated in this way can be used only for industrial applications, such as utility poles and railway ties, because the oil is irritating to human skin.
There is a need for lignocellulosic products that are dimensionally stable when exposed to moisture. There is a further need for lignocellulosic products that do not swell when immersed in water and that do not shrink when dried, and there is a need for applying a broad range of preservatives and organic biocides to wood and lignocellulosic composite products.